Amongst nucleic acid transfer systems, retroviral vectors hold substantial promise for gene therapy and other applications in which transfer of genetic material is desirable. These systems can transfer genes efficiently, and new vectors are emerging that are particularly useful for gene delivery to brain cells (Naldini et al., 1996 Science 272, 263).
There has been considerable interest in the development of retroviral vector systems based on lentiviruses, a small subgroup of the retroviruses. This interest arises firstly from the notion of using HIV-based vectors to target anti-HIV therapeutic genes to HIV susceptible cells and secondly from the prediction that, because lentiviruses are able to infect non-dividing cells (Lewis & Emerman 1993 J. Virol. 68, 510), vector systems based on these viruses are able to transduce non-dividing cells (e.g. Vile & Russel 1995 Brit. Med. Bull. 51, 12). Vector systems based on HIV have been produced (Buchschacher & Panganiban 1992 J. Virol. 66, 2731) and have been used to transduce CD4+ cells and non-diving cells (Naldini et al., 1996 Science 272, 263). However, in general, nucleic acid transfer efficiencies are not as high as with comparable murine retrovirus vector systems.
The HIV-based vectors produced to date result in an integrated provirus in the transduced cell that has HIV LTRs at its ends. This limits the use of these vectors as the LTRs have to be used as expression signals for any inserted gene unless an internal promoter is used. The use of internal promoters has significant disadvantages. For example, the presence of internal promoters can affect the transduction titres obtainable from a packaging cell line and the stability of the integrated vector.
Also, HIV and other lentiviral LTRs have virus-specific requirements for nucleic acid expression. For example, the HIV LTR is not active in the absence of the viral Tat protein (Cullen 1995 AIDS 9, S19). It is desirable, therefore, to modify or delete the LTRs in such a way as to change the requirements for nucleic acid expression. In particular, tissue specific gene expression signals may be required for some gene therapy applications. In addition, signals that respond to exogenous signals may be necessary. In murine retroviruses this is often achieved simply by replacing the enhancer-like elements in the U3 region of the murine lentiviral (MLV) LTR by enhancers that respond to the desired signals. This has not been feasible with viruses such as HIV because within the U3 and R regions of their LTRs are sequences, known as IST and TAR, which may inhibit gene expression and may or may not be responsive to Tat protein when heterologous, perhaps tissue specific, control sequences are inserted in the U3 region (Cullen 1995 AIDS 9, S19; Alonso et al., 1994 J. Virol. 68, 6505; Ratnasabapathy et al., 1990 4, 2061; Sengupta et al., 1990 PNAS 87, 7492; Parkin et al., 1988 EMBO. J 7, 2831). Even if the signals are responsive, it is undesirable to have to supply Tat as it further complicates the system and Tat has some properties of oncoproteins (Vogel et al., 1988 Nature 335, 606).
Parkinson's disease (PD) is a common neurodegenerative disorder that afflicts the growing population of elderly people. Patients display tremor, cogwheel rigidity and impairment of movement. It is generally thought to be an acquired rather than inherited disease in which environmental toxins, metabolic disorders, infectious agents and normal aging have all been implicated. PD is associated with the degeneration of nigrostriatal neurons which have their soma located in the substantia nigra. They send axonal projections to the basal ganglia and they use dopamine as their neurotransmitter. Some features of the disease can be controlled by the administration of L-DOPA, the metabolic precursor to dopamine, which diffuses across the blood brain barrier more effectively than dopamine itself. Unfortunately as the disease progresses the side effects of this treatment become unacceptable.
PD is an ideal candidate for gene therapy for several reasons. The clinical efficacy of systemic administration of L-DOPA suggests that restoration of neuronal circuitry is not essential for disease management. Therefore genetic manipulation of brain cells to provide local production of L-DOPA from tyrosine may be a realistic strategy for treatment. The biosynthesis of L-DOPA from tyrosine involves a single step suggesting that provision of tyrosine hydroxylase (TH) by genetic means may be sufficient and some success has been achieved using this strategy in small animals and in cell culture (Kaplitt et al., 1994 Nature Genetics 8, 148; During et al., 1994 Science 266, 1399; Horellou et al., 1994 Neuroreport 6, 49; Owens et al., 1991 J. Neurochem. 56, 1030). However, if one is to use local endogenous brain cells as L-DOPA factories for the treatment of PD in man it is likely that high levels of L-DOPA will be required to effect a treatment. These high levels must be efficiently converted to dopamine as the necessary neurotransmitter and primary therapeutic agent. It is likely therefore that it will be necessary not only to supply tyrosine hydroxylase but also DOPA decarboxylase (DD), the enzyme that converts L-DOPA to dopamine. This means that in a gene therapy strategy the genes for both of these enzymes will be required. However, it is clear from the literature that retroviral vectors achieve the highest titres and most potent gene expression properties if they are kept genetically simple (PCT/GB96/01230; Bowtell et al., 1988 J. Virol. 62, 2464; Correll et al., 1994 Blood 84, 1812; Emerman and Temin 1984 Cell 39, 459; Ghattas et al., 1991 Mol.Cell.Biol. 11, 5848; Hantzopoulos et al., 1989 PNAS 86, 3519; Hatzoglou et al., 1991 J. Biol. Chem 266, 8416; Hatzoglou et al., 1988 J. Biol. Chem 263, 17798; Li et al., 1992 Hum. Gen. Ther. 3, 381; McLachlin et al., 1993 Virol. 195, 1; Overell et al., 1988 Mol. Cell Biol. 8, 1803; Scharfman et al., 1991 PNAS 88, 4626; Vile et al., 1994 Gene Ther 1, 307; Xu et al., 1989 Virol. 171, 331; Yee et al., 1987 PNAS 84, 5197). This means only using a single transcription unit within the vector genome and orchestrating appropriate nucleic acid expression from sequences within the 5′ LTR. The need to express two enzymes from a single retroviral vector would require the use of an internal ribosome entry site (IRES) to initiate translation of the second coding sequence in a poly-cistronic message (Adam et al. 1991 J .Virol. 65, 4985). However, the efficiency of an IRES is often low and tissue dependent making this strategy undesirable when one is seeking to maximise the efficiency of metabolic conversion of tyrosine through to dopamine. The present invention addresses these problems.